Royston (Iceni) Weather Station

Daily Weather Observations - May 2004

May was yet another warmer than average month in Royston, but rainfall and sunshine were not far from normal.

In the 89 months since January 1997 no fewer than 70 including May 2004 have experienced above average temperatures. October 2003 was the last month with a below average mean temperature. The mean temperature in May this year was 1.0°C above average, although May 2003 and May 2002 were both warmer.

The temperature regime in May was very similar to May last year. The mean daily maximum temperature in May was 17.4°C, which is 0.6°C above the 30 Year (1973/2002) average for Royston. Due to the increased cloud cover at times (see note on sunshine below) the nights tended to be comparatively warmer compared to the average and the mean night minimum temperature was 1.4°C above average at 8.6°C.

The highest temperature recorded in May was 24.0°C on 17^th. This value is not particularly noteworthy for May, having been exceeded in 8 of the last 10 years, and in 15 years since 1972. The lowest temperature recorded in the month was 2.4°C on 22^nd. On this date the grass minimum temperature also fell to its lowest value of the month, 0.0°C. Although a ground frost was thus just avoided at Royston (Iceni) Weather Station on this occasion, reports from the lower lying parts in the north of the town indicate damage by frost to potato plants.

The mean earth temperature at 30 cms depth as recorded at 0900 GMT daily was 14.3°C. The lowest earth temperature recorded was 11.5°C on 5^th but by 20^th the earth temperature had risen to 17.1°C, its highest value of the month.

The first week of May was very wet but thereafter the month was relatively dry. The rainfall total for the month was 53.3 millimetres, which is 113% of the 1971/2000 average. It was the wettest May since 2002 only.

The number of "rain days" (0.2 millimetres or more) in May was 12, with rainfall exceeding 1.0 millimetre on 9 of these days. The wettest day of the month was 7^th with 20.7 millimetres rainfall. This was the wettest May day since 1992. The rainfall duration over the whole month was 45.1 hours.

Sunshine was at a premium in early May and by 12^th only 22.6 hours sunshine had been recorded. Matters improved considerably thereafter, however, and by 31^st a further 154.1 hours sunshine had been recorded. This brought the sunshine total for the month to 176.7 hours, but it was still not enough to reach the monthly average and in the event was 7% short of the 1971/2000 mean.

The sunniest day of the month was 19^th with 14.0 hours sunshine. Only 3 days (1^st, 8^th and 11^th) in the month were completely sunless, although 9^th came very close with only 3 minutes sunshine!

May was a relatively calm month and the highest wind gust speed recorded was 35 mph on 4^th. The highest daily (00/00 GMT) mean wind speed was 6.8 mph, also on 4^th, whilst the lowest daily mean wind speed was 0.2 mph on 26^th.

The analysis of wind directions as observed daily at 0900 GMT shows that on 20 days the wind contained a northerly component so it is perhaps surprising that May was a warmer than average month. The results are NORTH 7 days, NE 4, E 1, SE 0, S 4, SW 3, W 3, NW 9 and CALM 0.

The mean MSL air pressure in May was 1016.6 mB, which is very close to the average of the last 30 years. The highest and lowest pressure values in May were 1033 mB and 976 mB on 23^rd and 4^th respectively.

The mean relative humidity in May was 78.2%. The highest daily (00/00 GMT) mean relative humidity was 98.2% on 1^st whilst the lowest daily mean relative humidity was 60.6% on 24^th. The absolute lowest relative humidity reading in the month was 31% at 1410 GMT on 24^th.

Fog (visibility < 1000 metres) was not recorded on any day in May at the 0900 GMT observation hour.

This report concludes with a few summary statistics for the Spring (March/May) season. The mean daily maximum temperature in Spring was 14.2°C, which is 0.9°C above the 30 Year (1973/2002) average. The mean night minimum temperature was 6.1°C, which is 1.2°C above average. Total rainfall was 14% above the 1971/2000 average at 152.7 millimetres, whilst the Spring sunshine total of 424.1 hours was 96% of the 1971/2000 average. It was both the coldest and wettest Spring since 2001 only.

Met. Observer, Royston (Iceni) Weather Station

4^th June 2004

Return to Daily Weather Observations - May 2004 header page